Shearing heavy growths such as tree limbs on the order of two inches in diameter requires considerable force. Lopping shears for cutting such heavy growths typically comprise a pair of relatively long-handled levers, cooperating with opposed cutting members, such as a blade and anvil, a hook and blade or respective blades. Levering of the handles forces the cutting members together or, in the case of a passby lopper, past one another, to effect the severing action.
Lopping shears are often characterized by the use of extra long handles to provide increased leverage. Such handles must be sufficiently strong to withstand the forces involved in cutting heavy growths. However, solid handles, typically wooden, tend to be overly heavy and awkward. On the other hand, lighter tubular handles tend to lack the requisite strength, particularly at certain major stress points, or are prohibitively expensive. Thus, the length of handles tends to be limited by weight and cost factors.
In general, the use of compound leverage action mechanisms to increase leverage in tools is well known. Compound leverage action mechanisms have been employed, for example, in hook-and-blade-type pruning apparatus, in anviltype tools, and in shears. Examples of such pruning apparatus are described in U.S. Pat. Nos. 4,420,883 and 4,442,603 issued to E. Wallace, et al, on December 20, 1983 and April 17, 1984, respectively. Examples of compound action shears are described in U.S. Pat. Nos. 492,198, issued to C. Hamann on February 21, 1893, 2,384,822, issued September 18, 1945 to S. Drmic, 2,528,816, issued to H. Boyer on November 7, 1950, and 3,650,028, issued to G. LaPointe on March 21, 1972. Use of compound action leverage mechanisms in lopping shears is also known. An example is described in U.S. Pat. No. 3,372,478, issued to E. Wallace, et al, on March 12, 1968.
However, known compound action leverage mechanisms, when used in such lopping shears, and particularly passby lopping shears, tend to be disadvantageous in a number of respects. The action mechanisms tend to be relatively complicated, and render difficult adjustment of blade tension and removal of the blade for sharpening. Further, torques applied to the handles in such shears tend to be transmitted to the blades of the shears, causing the blades to twist with respect to each other, impeding the cutting function and tending to overload and damage the blade pivot.